1. Field of the Invention
The invention relates to a color solid-state image pickup device of single chip type and to a digital camera equipped with the color solid-state image pickup device.
2. Description of the Related Art
A typical color representation means employed in a solid-state image pickup device such as a CCD or CMOS types consisting of color representation means (i.e., a color solid-state image pickup device of single chip type) in which micro-color filters having three or four different spectral characteristics are arranged, in a mosaic pattern, on respective photodiodes (pixels) arranged on the surface of an image pickup element in the form of a two-dimensional array.
FIG. 38A is a conceptual plan view of the CMOS image sensor of single chip type as a related-art. A known image sensor 101 has a plurality of photodiodes 102 arranged on the surface of the sensor in a two-dimensional arrayed pattern. Micro-color filters having three or four different spectral characteristics (micro-color filters of primary colors, e.g., R, G, and B) are arranged, in a mosaic pattern, on the surfaces of the respective photodiodes 102.
The micro-color filters are roughly classified into two types: that is, primary colors (R, G, B) and complementary colors (G, Ye, Cy, Mg). According to a method of arrangement, a stripe pattern and a Bayer pattern such as disclosed in U.S. Pat. No. 3,971,065 or U.S. Pat. No. 5,063,439 are proposed. However, the photodiodes are discretely spread over a two-dimensional plane, and the respective photodiodes correspond to any of three or four types of different spectral sensitivities. Hence, the following problem arises.
(1) When an image having a spatial frequency higher than the Nyquist frequency determined by a pitch between the photodiodes is projected on a color solid-state image pickup device, the high spatial frequency component is folded toward a low-frequency range, thereby resulting in generation of a false signal called aliasing or a Moiré and deteriorating the quality of a photographed image.
(2) Color filters having different spectral sensitivities correspond to different locations on a two-dimensional plane. Nyquist domains (i.e., spatial frequency distributions) of respective colors fail to coincide with each other by means of distribution patterns of respective color filters. Consequently, a phenomenon called a false color or a color Moiré is induced, thereby deteriorating the quality of the photographed image.
For these reasons, as a method for lessening Moiré, there has hitherto been adopted a method for inserting an optical low-pass filter (OLPF) between a light-gathering optical system (i.e., a lens system) and the image sensor. Use of this method enables attenuation of a high spatial frequency component, thereby improving Moiré.
However, the effect of improving Moiré by use of the optical low-pass filter is insufficient. The optical low-pass filter is constituted by combination of birefringent optical materials, such as quartz plates. A structure in which an optical low-pass filter is attached to a package for protecting an image sensor suffers a problem of a hike in manufacturing costs as well as a problem of the optical low-pass filter being likely to be broken by mechanical stress inflicted on the package.
Meanwhile, a recent image sensor has undergone miniaturization of a photodiode; i.e., an increase in the number of pixels. Hence, the resolution of the image pickup device has been improved by leaps and bounds. The Nyquist frequency is also made higher as a result of miniaturization of the pitch between photodiodes. Therefore, in principle, occurrence of Moiré tends to be inhibited.
However, in order to integrate a plurality of photodiodes such as millions of pixels on one chip, other problems have come up as follows.
First, in association with miniaturization of the photodiode, a microlens must also be miniaturized. In this case, the relative thickness of the microlens becomes larger, thereby shortening the focal length of the microlens. As a result, focus is achieved at an elevated point forward of a photodiode. In order to adjust the focal length, another lens (inter-layer lens) must be formed into a lower portion of the microlens. Formation of another lens makes the structure of the microlens complicated, thereby posing difficulty in achieving stable manufacturing yield.
A light-shielding film having apertures is stacked over photodiodes and peripheral circuits thereof such that apertures come to positions above the respective photodiodes. FIG. 38B is a plan view showing a light-shielding film for four pixels denoted by broken lines “b” shown in FIG. 38A. The light-shielding film 103 has the apertures 103a corresponding to the respective photodiodes 102. The apertures 3a formed in the light-shielding film 103 are miniaturized in association with miniaturization of the photodiodes. When the aperture has a size of, e.g., 1 μm or less, the intensity of incident light is greatly attenuated when the incident light passes through the aperture, depending on the wavelength of the light. For example, red (R) light has a wavelength of about 0.650 μm. Hence, when the aperture has a size of 1 μm or less, wave optical effects must be taken into consideration.
Therefore, when an attempt is made to miniaturize the photodiodes remaining in a related-art arrangement; that is, dispersed on a per-color basis, miniaturization of photodiodes and a read circuit, which are to be fabricated on a semiconductor substrate, and miniaturization of an on-chip light-gathering optical system (e.g., microlenses, color filters, and apertures of a light-shielding film) simultaneously become indispensable. The intensity of the light entering a light-receiving section is significantly decreased by means of the wave optical effect. Even if an image of a bright object is captured, a problem of insufficient sensitivity will arise.
FIG. 39A is a cross-sectional view of a related-art CMOS image sensor. A photodiode 102 is fabricated on the surface of a semiconductor substrate. Light-shielding films 103 are formed at positions above the photodiodes 102. Color filters 104, such as R, G, and B color filters, are formed at positions above the light-shielding films 103. Microlenses (top lenses) 105 are formed on the color filters 104. Peripheral circuit sections 196 are provided beside the respective photodiodes 102, and the peripheral circuit sections 196 are shielded by the light-shielding films 103.
FIG. 39B shows an equivalent circuit of one photodiode and a peripheral circuit thereof. The photodiode 102 is connected to a transistor 196a constituting a source follower amplifier and to a transistor 196b constituting a reset gate. These transistors 196a, 196b are fabricated around the photodiode 102.
In the case of a CMOS image sensor, wiring electrodes provided for each photodiode 102, such as an X address line, a Y address line, a power line, and a reset signal line, must be formed in positions which are located above the photodiode 102 and avoid a light-receiving surface of the photodiode 102. Therefore, the wiring electrodes are laid in a grid pattern so as to avoid respective light-receiving surfaces of the photodiodes.
Laying all the wiring electrodes within a single plane while preventing electrical contact is not possible. In an illustrated example, signal lines are arranged in a three-layer structure as indicated by means of signal lines 106, 107, and 108. The signal lines 106, 107, and 108 are isolated from each other by means of an interlayer insulation film. Hence, a distance “a” between the surface of the photodiode 102 and the top lens 105 eventually becomes longer. Even when an increase in the number of pixels is pursued, the distance “a” cannot be shortened. Hence, the light entering the microlenses 105 reaches the respective photodiodes 102 while passing through narrower channels in the case of an image sensor for which an attempt has been made to increase the number of pixels.
Next, there occurs a phenomenon in which a difference arises between the center of a light-receiving area and a peripheral area of the CMOS image sensor in terms of sensitivity and color reproducibility. The phenomenon is a so-called (brightness, color) shading phenomenon. In particular, as the light-gathering optical (i.e., a camera lens) system is miniaturized and, eventually, the focal length becomes shorter, sensitivity variations, which are attributable to an increase in difference between the center and neighborhood of the light-receiving area and surroundings of the same in terms of incident angle of the incident light, cannot become negligible.
The following manners are proposed for solving this problem.
(1) The arrangement of the microlens 105 is displaced toward the center by a predetermined amount with increasing proximity to the peripheral area of the image sensor.
(2) Another microlens (i.e., an in-layer lens) is provided below the microlens 105 (also called a “top lens”). The light gathered by the top lens is again positioned and converged on the respective photodiodes through use of the in-layer lens.
(3) Sensitivity variations are electrically corrected by a peripheral signal processing circuit (external circuit).
In association with miniaturization of the photodiodes, the improvement means described in (1), (2) encounter difficulty in controlling the shape of a highly accurate microlens and the arrangement of the microlenses. Hence, adoption of the improvement means (1), (2) has been difficult. In the case of the CMOS image sensor, presence of multilayer wiring sections 106, 107, and 108 and the interlayer insulation film poses extreme difficulty in forming an in-layer lens. Moreover, in relation to all the countermeasures (1), (2), and (3), if a difference arises in a lens system (e.g., a difference between a lens system of a digital still camera and a lens system of a built-in camera of a portable cellular phone), the appearance of a shading phenomenon also changes. This requires each image pickup system for a certain design change before said improvement means are applied.
In relation to the related-art color solid-state image pickup device involving these problems, such as disclosed in U.S. Pat. Nos. 5,965,875, 4,438,455, or JP-A Hei 1-134966, proposes a CMOS image sensor, wherein a photodiode for detecting a blue color, a photodiode for detecting a green color, and a photodiode for detecting a red color are formed so as to overlap each other in a depthwise direction of a semiconductor substrate. The CMOS image sensor utilizes a principle described in “A Planar Silicon Photosensor with an Optimal Spectral Response for Detecting Printed Material” written by Paul A. Gary and John G. Linvill, IEEE Transactions on Electronic Devices, Vol. Ed-15, No. 1, January 1968; that is, the principle of the photoelectric conversion characteristics of respective photodiodes having wavelength dependency (spectral sensitivity) depending on the depth of a p-n junction of each photodiode from the surface of a semiconductor substrate.
The CMOS image sensor, such as disclosed in U.S. Pat. No. 5,965,875, utilizes a correlation existing between absorption of light in a depthwise direction of each pixel or photodiode and the wavelength of visible light. The Nyquist domain does not change according to the colors (R, G, and B). Hence, a false color or color Moiré is considered less likely to arise.
However, spectra of respective color components are determined by wavelength dependence of photoelectric conversion efficiency with reference to the depth of the light having entered the silicon substrate. Further, ohmic contacts are provided for photodiode structures compatible with the light rays of different wavelengths, thereby reading an electric signal directly outside. As a result, there arises a problem of a relative decrease arising in the area of the light-receiving section of the photodiode. Moreover, there also arises a problem of a necessity for laying multilayer metal wiring on the surface of the image pickup element in the X and Y directions.